This invention relates to an electroluminescence device (thin-film EL device) including a pair of electrodes for voltage application and having an insulating layer formed along and on one side of an EL emission layer or a fluorescent layer.
Widely known is a thin-film EL device of a double layer insulation structure which has been manufactured through the formation of a thin film made of fluorescent material, such as ZnS:Mn, by a means of an electron beam evaporation or a sputting method and the formation of a thin insulating layer, such as Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5, BaTiO.sub.3, PbTiO.sub.3, SrTiO.sub.3, Y.sub.2 O.sub.3 or Sm.sub.2 O.sub.3, at each side of the fluorescent film.
The thin-film EL devices have recently been used for a display for industrial measuring instruments and lap-top personal computers, in view of their excellent visual recognition property.
As an insulating material for thin-film EL elements use is made of one having a high relative performance and high luminance in view of the withstand voltage property, relative permittivity, film formation property, property of a close bond to the associated film.
Among various insulating materials, Y.sub.2 O.sub.3 is used as such an insulating material as disclosed in IEEE-Transactions on Electron Devices Vol. ED-31, No. 1, Jan. 1984 and Sm.sub.2 O.sub.3 is employed as such an insulating material as disclosed in J. Applied Phys. Vol 21, 1982, P 1028.
However, the conventional thin-film EL devices have various drawbacks as set out below.
Al.sub.2 O.sub.3, if employed as such an insulating material, can relatively readily be formed, as a single film, on the associated film structure by means of a sputtering or an electron beam evaporation method, but if formed as an EL layer on, for example, a ZnS, CaS or SrS film structure, Al.sub.2 O.sub.3 manifests a poor affinity for these materials and, at the time of film formation, the peeling of one film from the associated film. Even if no peeling takes place at that time, it does occur due to the light emission aging and the consequent poor bonding force involved. In order to eliminate such a drawback, an attempt is made to interpose an intermediate layer, such as Si.sub.3 N.sub.4, between the Al.sub.2 O.sub.3 layer and the ZnS layer, requiring more manufacturing steps and more complicated steps and hence a higher cost.
Furthermore, the use of Al.sub.2 O.sub.3 does not necessarily assure any adequate threshold voltage nor high luminance at the initial luminance level.
Upon the light emission aging, the device reveals a greater (i.e. 60 to 70 V) shift of the emission start voltage toward a high voltage side in unstable fashion.
In the case of Ta.sub.2 O.sub.5, the device shows almost no shift of emission start voltage resulting from the emission aging, but is somewhat lower in its luminance level and, moreover, is liable to produce dielectric breakdown.
In addition to this, if the dielectric breakdown mode, though dependent from the manufacturing method, is of a propagation type, it is propagated into not only one spot in the device but also many other parts. As a practical solution to this problem a composite SiO.sub.2 layer structure is employed, but the manufacturing steps are very complicated and higher in costs.
If Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5 is formed as a film on the ITO transparent electrode, it is liable to lose its transparence due to the development of color in that film.
When use is made as an insulating layer of BaTiO.sub.3, PbTiO.sub.3, Y.sub.2 O.sub.3 or Sm.sub.2 O.sub.3, then it is very difficult to control the composition of the insulating materials. In this case, more time and more manufacturing steps are required in the setting of the parameters in an attempt to form films of better quality because of a greater fluctuation among the parameters.